Suppress canvas-alpha-bug failure on Mac Intel Haswell GPUs.

conformance/rendering/canvas-alpha-bug.html is failing on this
configuration on 10.13 and later. Rather than specialize the
suppression for multiple OS versions, suppress it on this GPU type.

Tbr: jdarpinian@chromium.org
No-Try: True
Bug: 886970
Change-Id: Ifeeaa59aca1c232b3d3eda6b2a82b35451c8a2bc
Reviewed-on: https://chromium-review.googlesource.com/c/1298199
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602479}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -789,6 +789,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['mac', 'amd'], bug=642822)
 
     # Mac Intel
+    self.Fail('conformance/rendering/canvas-alpha-bug.html',
+        ['mac', ('intel', 0x0a2e)], bug=886970)
     self.Fail('conformance2/rendering/framebuffer-texture-level1.html',
         ['mac', 'intel'], bug=680278)
     self.Fail('conformance2/textures/misc/angle-stuck-depth-textures.html',

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -602,9 +602,12 @@ class WebGLConformanceExpectations(GpuTestExpectations):
     #     ['mac', 'amd', 'release'], bug=735483)
 
     # Mac Intel
+    self.Fail('conformance/rendering/canvas-alpha-bug.html',
+        ['mac', ('intel', 0x0a2e)], bug=886970)
     self.Fail('conformance/rendering/rendering-stencil-large-viewport.html',
         ['mac', 'intel'], bug=782317)
 
+
     # Mac Retina NVidia failures
     self.Fail('conformance/attribs/gl-disabled-vertex-attrib.html',
         ['mac', ('nvidia', 0xfe9)], bug=635081)